Field of the Invention
The present invention relates to an optical gas concentration measuring method related to the concentration of a predetermined chemical component in a gas, and a method for monitoring the gas concentration by the method.
Description of the Background Art
In the manufacture of a semiconductor, mixed gases are often supplied from the same line inside a processing chamber of a semiconductor manufacturing device. The supply of such a mixed gas requires that a mixture ratio of component gases be kept constant during the treatment process period, and instantaneously changed as intended. To this end, a flow rate control device, such as a flow control system component (FCSC), for example, that comprises a gas flow rate measurement mechanism and a gas flow rate adjustment mechanism is arranged in the gas supply line. In this FCSC, the degree to which the flow rate per unit time (hereinafter also referred to as “unit flow rate”) of each component gas that constitutes the mixed gas can be accurately measured is important.
Today, in a semiconductor manufacturing process in which there are many opportunities to implement a treatment process such as film formation or etching at an atomic- to nano-order level, the unit flow rate of each component gas in a mixed gas immediately prior to introduction to a processing chamber needs to be measured accurately and instantaneously down to a small range.
In a conventional flow rate control device, generally the flow rate of a single component gas prior to mixture is measured and the target mixture ratio of the mixed gas is calculated from the measured flow rate value.
Nevertheless, the mixture ratio of the mixed gas at the moment of introduction into the processing chamber (hereinafter also referred to as “actual mixture ratio”) is not always guaranteed to be the same as the mixture ratio calculated from the measured flow rate values (hereinafter also referred to as “measured mixture ratio”) during process execution. Thus, conventionally a feedback mechanism is provided that measures the flow rate of each single component gas either continually or at a predetermined interval, and adjusts each of the flow rates so that, when the flow rate of any single component gases fluctuates, the mixture ratio becomes the original predetermined mixture ratio based on the new value (Patent Document 1, for example).
On the other hand, examples of a gas concentration measuring system include a system that uses a partial pressure measurement sensor that measures the partial pressure of a material gas by a non-dispersive infrared absorption method, and calculates the concentration of the material gas on the basis of the partial pressure measurement value of this sensor by a mathematical operation (Patent Document 2, for example).
Further, in metal organic compound chemical vapor deposition (MOCVD; chemical vapor deposition that uses a metal organic compound) as well, formation of a uniform film requires control of the supplied concentration of the metal organic compound so that the supplied concentration of the metal organic compound is constant during the film formation process period, or so that the supplied concentration fluctuates in accordance with the component distribution of the metal organic compound to ensure formation of a film with a preferred component distribution. Generally, the metal organic compound is mixed into a carrier gas via bubbling or the like, and supplied to the processing chamber. The used metal organic compound is not limited to a single compound, and a plurality of compounds may be used as well. Examples of the method used to supply the raw material gases of a plurality of types of metal organic compounds in accordance with design values include a method for using infrared gas analysis means (Patent Document 3, for example).
Further, in a system in which bubbling is used, problems such as the following also arise (Patent Document 4).
During film formation, a predetermined number of organic metal gases are supplied from a secondary side of each of a plurality of bubbling vessels to a switch valve, the organic metal gases combined by the switching of the switch valve are supplied to a reactor, and the film is formed.
For example, during InGaP film formation, organic metal supply lines, such as a trimethylindium (TMI) supply line, a trimethylgallium (TMG) supply line, a phosphine (PH3) supply line, and an arsine (AsH3) supply line, supply organic metals to the reactor, and a film is formed by a metal organic chemical vapor deposition (MOCVD) method.
In a semiconductor laser of a compound semiconductor (for GaAs substrate usage), an LED, or the like, a multilayered film is formed using an MOCVD method. When a multilayered film is formed by a conventional vapor deposition device of a compound semiconductor, a certain film layer is formed and then the organic metal gases for the next film formation are supplied to the reactor by opening and closing a plurality of switch valves. When a valve is opened, however, the flow rate to the reactor suddenly becomes excessively high, resulting in the occurrence of time delays until stabilization to a specified flow rate, adversely affecting the consistency of the formed film thicknesses.
In addition, to ensure that the internal pressure of the reactor is constant, the total flow rate inside the reactor must be constant during film formation. That is, when the internal pressure of the reactor is not constant, fluctuation in internal pressure causes inconsistencies in film thickness. As a result, while the organic metal gases are supplied at specified flow rates during film formation of a certain layer, a carrier gas for flow rate compensation is supplied at a required rate by a mass flow controller when the flow rate does not satisfy a predetermined total flow rate inside the reactor.
Nevertheless, while the flow rate of the carrier gas for flow rate compensation changes when the specified flow rate of an organic metal gas differs from that during all film formations, the flow rate change performed by the mass flow controller may incur a time delay until the flow rate stabilizes at the new value, resulting in a time delay until the internal pressure of the reactor becomes constant, thereby adversely affecting the consistency of film formation. The time delay until the compensatory carrier gas flow rate reaches a constant flow rate may cause an abruptness between film formations to worsen, adversely affecting the characteristics of the produced semiconductor.